1. Field of the Invention
The present invention relates to a reader/writer that communicates with a radio communication terminal including short-distance radio communication function termed a non-contact IC card or the like, and to a communication method of the reader/writer.
2. Description of the Related Art
Recently, a non-contact IC card has been widely used for a train ticket, membership card, employee ID card, payment settlement tool at stores, or the like. Such non-contact IC card performs radio communication with a reader/writer in close proximity thereto and performs authentication processing. Therefore, non-contact IC cards can be used while being carried in a wallet, pass case or the like, and so it is convenient for a user compared to the case in which a magnetic card or other cards are used.
On the other hand, there is proposed a non-contact IC card (or a circuit component having function equivalent to the IC card) incorporated into a portable electronic device such as a mobile phone unit in which similar authentication and payment settlement can be performed using such devices. Note that, in the case where an IC card function unit is assembled into a portable terminal, the unit is not necessarily card-shaped; however, if it is termed “IC card” in the following explanation a unit having function of IC card is included unless there is a specific explanation. Further, a non-contact IC card of this kind is also called “RFID (Radio Frequency Identification)”, “Radio IC Tag” or the like. Although various kinds of shape such as a label, coin and stick are conceivable if those are each used as a single unit, those are called IC cards in this specification for convenience.
In the case of performing radio communication with a reader/writer, an IC card function unit is operated by electromagnetic induction of the reader/writer. In other words, tuning processing of a carrier wave of a predetermined frequency output from the reader/writer is performed on the IC card side and the detected carrier wave is modulated by ASK (Amplitude Shift Keying) or other methods, and then data is transmitted to the reader/writer side. The ASK modulation is an amplitude modulation in which a digital signal is represented by a difference in the amplitude of a carrier wave.
FIG. 1 is a schematic diagram showing a state of communication between a reader/writer and an IC card. As shown in this figure, a reader/writer 1 includes a loop antenna 2 to transmit a carrier wave F1(x) of a predetermined frequency. An IC card 3 also includes a loop antenna 4 to transmit a response waveform F2(x) on which ASK-modulation was performed by the loop antenna 4. The loop antenna 2 in the reader/writer 1 receives a composite wave of the carrier wave F1(x) and the response waveform F2(x).
Japanese Published Patent Application No. 2003-67693 (FIG. 2) discloses a non-contact IC card.